The subject matter disclosed herein relates to magnetic resonance imaging (MRI), and more specifically, to systems and methods for producing MRI data with free induction decay (FID) and gradient echo data.
Generally, MRI images are produced from data that assess the interactions between magnetic fields and the nuclear spins of certain gyromagnetic materials, such as hydrogen nuclei, of the object being imaged. The imaged object may be immersed in a strong primary magnetic field, and subject to magnetic field gradients. In some applications, a radiofrequency (RF) magnetic field pulse may excite the gyromagnetic nuclei and move it from a basal state to an excited state. With the removal of the RF magnetic field pulse, the gyromagnetic nuclei may return to the basal state in a relaxation trajectory. During the relaxation, the gyromagnetic nuclei may emit RF signals that can be measured by an acquisition coil.
Magnetic field gradients may be applied to the object during the imaging process. Due to a relationship between the angular speed of the precession of a gyromagnetic material and the strength of the magnetic field, a magnetic field gradient may generate a spatial distribution of precession speeds. This effect leads to a spatial encoding of the RF signals emitted by the gyromagnetic nuclei. The spatial location of a given component of the acquired data may be determined based on the emission frequency of that component. Each spatial encoding may correspond to a region of a frequency space diagram, also known as a k-space diagram. Multiple gradients may be employed to cover more regions of the k-space diagram.
The data measured may be used to generate an image of the object. In some applications, the image is a map of the distribution density of the gyromagnetic nuclei. In other applications, the image may be a map of the distribution of the time constants related to the relaxation of the component of the nuclear magnetization parallel to the primary magnetic field, known as a T1-weighted image. In other applications, the image may be a map of the distribution of the time constants related to the relaxation of the components of the nuclear magnetization perpendicular to the primary magnetic field, known as T2-weighted image. In some applications, the image may be a map of the distribution of the effective time constants related to the relaxation of the components of the nuclear magnetization perpendicular to the primary magnetic field, known as T2*-weighted image.
To quantify T1, T2, or T2*, data is generally collected at least at two different time points of the relaxation process. Quantification of T1, T2 and T2* relaxation time may be used to produce T1, T2 and T2* maps of the object. In some situations, the time points may be determined based on a time interval called echo time (TE), which is the time between the RF magnetic field pulse and the time when the components of the nuclear magnetization perpendicular to the primary magnetic field align to produce an “echo” signal.